Trump has now responded to the Raging Bull actor, labelling him “punch-drunk”.

He tweeted: “Robert De Niro, a very Low IQ individual, has received to many shots to the head by real boxers in movies. I watched him last night and truly believe he may be ‘punch-drunk’.

“I guess he doesn’t realize the economy is the best it’s ever been with employment being at an all time high, and many companies pouring back into our country. Wake up Punchy!”

Earlier this week, De Niro apologised to Canadians for the “idiotic behaviour of my president” after Trump branded Prime Minister Justin Trudeau “dishonest” and “weak” following the Group of Seven summit.

Trump has been in Singapore for a historic meeting with North Korean leader Kim Jong Un.